Non-volatile memory systems, such as flash memory, have been widely adopted for use in consumer products. Flash memory may be found in different forms, for example in the form of a portable memory card that can be carried between host devices or as a solid state disk (SSD) embedded in a host device. When writing data to a conventional flash memory system, a host typically writes data to, and reads data from, addresses within a logical address space of the memory system. The memory system then commonly maps data between the logical address space and the physical blocks or metablocks of the memory, where data is stored in fixed logical groups corresponding to ranges in the logical address space. Generally, each fixed logical group is stored in a separate physical block of the memory system. The memory system keeps track of how the logical address space is mapped into the physical memory but the host is unaware of this. The host keeps track of the addresses of its data files within the logical address space but the memory system generally operates without knowledge of this mapping.
A drawback of memory systems that operate in a logical address space is fragmentation. Data written by a host file system may often be fragmented in logical address space, where many fixed logical groups are only partially updated with new data. The fragmentation may occur as a result of cumulative fragmentation of free space by the host file system, and possibly even as a result of inherent fragmentation of individual files by the host file system. Data previously written may become obsolete due to deletion and cause further fragmentation. The fragmented logical groups will need to be rewritten in full in a different physical block. Rewriting the fragmented logical groups may involve copying unrelated data from the prior location of the logical group, a process known as garbage collection. As fragmentation increases, the transfer speed of a memory system may decrease.
To provide a measure of memory system metrics, tests may be applied to the memory system to gauge the write performance of the memory system. For example, memory systems may have transfer speed specifications that can be rated by protocols such as the Speed Class specification from the SD Association, for example. The Speed Class specification includes Speed Classes 2, 4, and 6, which denote the guaranteed minimum transfer speeds for memory systems of 2, 4, and 6 MB/s, respectively. A Speed Class test run on a memory systems measures metrics such as the sequential write performance of an addressable unit (Pw), the time for a file allocation table (FAT) write (Tfw), and the rolling average of time for multiple FAT writes (Tfw(avg)). The Speed Class test run may also measure the performance of the memory system with fragmentation (Pc), which includes the corresponding read performance measurements. Garbage collection activity occurring during a Speed Class test can adversely affect the test results.